Ferrite circulators have been used for many years in the UHF and microwave frequency range. Such a circulator is a three port device consisting of ferrite material, one or more magnets, three short transmission lines and a central conductive junction. A circulator typically functions as an isolator to substantially reduce power transfer between two of the ports.
In a typical application, power enters port 1 of the circulator and is rotated to emerge at port 2. Power entering port 2 emerges at port 3 and the power entering at port 3 emerges at port 1. In most applications, one of the ports is terminated with a fixed load resister.
The magnets within a ferrite circulator provide a fixed bias magnetic field which is applied to the ferrite material and central conductive junction. Circulators are tuned to operate at selected frequency ranges. A primary factor in the tuning of a circulator is the intensity of the magnetic field which is applied to the ferrite material. A method for adjusting the intensity of this magnetic field is presented in U.S. Pat. No. 3,935,549 to Jachowski.
A principle problem encountered in the use of ferrite circulators is the buildup of heat which is produced when the microwave energy is dissipated within the various elements of the circulator. As the temperature of the circulator increases due to heat buildup, the tuned frequency of the circulator can also change. Thus, a circulator which is tuned at a relatively low temperature can be driven out of its tuning condition as heat is built up within the circulator. When a circulator is operated at an out of tune condition, there is greater dissipation of the signal that is transmitted through the circulator. This reduces the effective power which should be transferred through the circulator.
The problem of heat buildup in circulators has been addressed in several prior patents. U.S. Pat. No. 3,621,476 discloses a circulator which has a heat sink and U.S. Pat. No.3,452,298 discloses a circulator which includes temperature compensating gyromagnetic material.
In view of the problem of heat buildup within ferrite circulators and the resulting detuning of the circulators, there exists a need for a temperature compensated circulator which can operate over a broad temperature range without substantially detuning the circulator.